In the refining of crude oil cracking is a widely used operation. Cracking is a method to obtain lighter products from a relatively heavy feedstock. Cracking operations include thermal cracking, catalytic cracking and hydrocracking. After the cracking operation the cracked products are separated, generally by distillation, in at least one distillate fraction and a residual fraction. This latter fraction is frequently used as a fuel oil component.
This residual fraction, however, contains several relatively light hydrocarbons which have a higher intrinsic value than just that of a fuel oil component. This is especially the case in residual fractions obtained after hydrocracking and catalytic cracking operations. These relatively light hydrocarbons are the main reason why these residual fractions are unfit for use in bitumen compositions. So, it would appear that separation of these relatively light hydrocarbons would be beneficial since then not only relatively valuable hydrocarbons would be obtained, but also a fraction suitable for use as bitumen component.
Separation of these relatively light hydrocarbons from the residual fraction by vacuum distillation appears to be troublesome, since fouling and plugging problems may arise. These problems are due to the fact that at the desired distillation conditions a big proportion of the residual fraction evaporates thereby entraining heavier products. The latter products not only cause a relatively bad separation but may also cause plugging problems in discharge conduits at the top of the distillation column. The bottom fraction of the distillation may give rise to troubles, too, since fine particles of the catalyst, applied to catalytic and/or hydrocracking, which are present in the residual fraction, are concentrated in the bottom fraction of the vacuum distillation, which tends to be rather viscous, and causes fouling of the conduit system for removing this bottom fraction. The present invention provides a solution to these problems.